The present invention relates to an overcurrent detection circuit for suitably detecting an overcurrent in a power supply circuit for supplying electric power from a power supply to a load in which a rush current may be generated.
Conventionally, in a power supply circuit for supplying electric power from a battery to a load such as an electric equipment installed in a car, a protection means has been used for protecting the load or electric wires from an overcurrent. For example, Postexamined Japanese Patent Publication 8-14598 discloses a protection means, in which two stages of overcurrent detection lines are provided by use of a semiconductor switch with an electric current detection function (Intelligent Power Switch) or the like, so that a load such as a lamp in which a rush current may be generated can be protected from an overcurrent.
Generally, a conductor of an electric wire is formed by twisting strands. In the case of investigation on the detection of an overcurrent in such an electric wire, it is necessary to investigate not only the case where the twisted wire without fraying is coated with insulating coating resin entirely, but also the case where the insulating coating is broken partially and the strands fray partially so as to appear outside so that the strands may come into contact with other conductors such as a grounded line, etc.
In the case where a coating is broken as mentioned above, a current is concentrated on fewer strands in the conductor, so that the temperature rising of the conductor becomes larger than that in the above-mentioned case where the coating is complete even if an overcurrent is at the same level. Accordingly, because of heating of the conductor, the coating overheats in a shorter time so that the heat-aging of the coating resin is accelerated.
In the worst case, only one of the strands constituting the twisted wire frays and comes outside so as to be in contact with a grounded line. FIG. 6 shows the heat-aging characteristic of an electric wire. The heat-aging characteristic indicates a conduction time taken for heat-aging of coating resin to begin to progress suddenly when a certain value of current is being applied to an electric wire. The heat-aging characteristic A1 at the time of one strand short-circuits is shifted to the lower current side than the heat-aging characteristic A2 in the condition of complete coating.
In such a case, it is difficult to cope with the one-strand heat-aging characteristic A1 only by two-stage overcurrent detection based on detection characteristic A3 shown by one-dot chain line in FIG. 6 as disclosed in the above-mentioned JP 8-14598.
In order to perform two-stage overcurrent detection, therefore, it is necessary to shorten the initial detection time T for large current detection in view of the one-strand heat-aging characteristic A1 and normal load current characteristic A4. Alternatively, it will go well if the detection characteristic has a number of stages such as three-stage detection characteristic A5 shown by broken line in FIG. 6.
In order to shorten the detection time T for large current detection, however, it is necessary to count the detection time T with high accuracy. On the other hand, in order to perform multi-stage overcurrent detection, there is a problem that the number of parts constituting the circuit increases so that the circuit becomes large in size.